BTRFS uses COW... what is the resolution of COW? [SOLVED]

Perhaps my google-fu is poor since I found nothing relevant while searching. What is the true resolution of BTRFS's COW? In other words, does a log file being written to to a BTRFS filesystem have growing copies of itself on the character level or the blocksize level?

Character level = a copy of the file is saved after each character is written to it.Blocksize level = a copy of the file is saved after it grows to the size of the BTRFS block size (which I believe is called a leafsize in BTRFS terms).